The verdict in the Kay Gilderdale case is further evidence that the law on mercy killing is out of date, experts say, and unable to deal properly with public views on compassionate death and assisted suicide.

Gilderdale pleaded guilty to assisting suicide, an offence under repeated scrutiny since the case of Debbie Purdy, the multiple sclerosis sufferer who forced prosecutors to produce guidelines on when relatives of the terminally ill are likely to face prosecution for assisting the death of loved ones.

Although the sentence for assisting suicide is up to 14 years under the Suicide Act 1961, Gilderdale was given a 12 month conditional discharge, walking free from court today. Her acquittal on a charge of murder, after the jury indicated strong sympathy for her case, is in strong contrast to other cases.

Last week another mother, Frances Inglis, was jailed for nine years for murder after injecting her son with a lethal dose of heroin – regarding as a particularly punitive sentence in circumstances where she said she acted out of compassion for his brain damage.

The cases highlight the confusion surrounding the legal tests for murder, assisted suicide and euthanasia. Campaigners argue there should be clear guidance on how prosecutors should respond to cases where a competent adult requests help to die, and where relatives inflict death without the clear wishes of their loved ones. "The law needs to protect potentially vulnerable people by being tough on malicious or irresponsible behaviour, but it also needs to be flexible enough to show mercy when the motivation is clearly compassion", said Sarah Wootton, chief executive of campaign group Dignity in Dying.

The director of public prosecutions has outlined 16 "public interest factors" in favour of a prosecution and 13 factors against in assisted suicide cases, after Purdy's challenge in the House of Lords.

The guidelines help families to know in advance whether they are likely to be prosecuted for assisting in suicide abroad, prosecutors say – but are of little use when there is a potential murder charge. This had led to further calls for separate rules surrounding "mercy killing", described as having a "clear ethical difference" to assisted suicide.

But the clear difference between Gilderdale's case and Inglis's lies in the facts, lawyers have said. Unlike Inglis's son, who had made no sign he wished to die, Gilderdale's daughter had attempted suicide and made an "advance decision" – a legal document which allows ill individuals to refuse life-sustaining treatment in advance.

• This article was amended on 25 February 2010. The original stated that Kay Gilderdale was given a suspended sentence. This has been corrected.